In a ground survey and investigation of damage inside of an existing structure or the like, a non-destructive measurement for measuring the characteristics of a medium without destroying the medium to be surveyed is performed. Japanese Unexamined Patent Application Publication No. 2004-163322 discloses a method for measuring the characteristics of a medium by installing a plurality of seismic vibrators in contact with the medium, receiving signals transmitted from these seismic vibrators with a receiver, and analyzing the signals. In this method, each seismic vibrator transmits a seismic wave based on a pseudo random code signal of a different pattern, and so, in the receiver, a specific seismic wave is separated from a synthetic seismic wave in which the seismic waves transmitted from the plurality of seismic vibrators are synthesized.
In such a method, because the transmission energy of a seismic source is smaller than the seismic source such as dynamite, a process that is a so-called stack, which transmits and receives the same signal a plurality of times, is used in order to draw a distinction between random background noise and the signal from the seismic source. In such an application, a long measuring time is required, and so a technique for simultaneously outputting seismic waves from a plurality of seismic sources as disclosed in Patent Document 1 has the potential to streamline the operation.
In the conventional method, the receiver calculates a correlation value between i) a pseudo random code signal that is the same as the pseudo random code signal used by the seismic vibrator and ii) a reception signal based on a synthetic seismic wave in which seismic waves received from a plurality of seismic vibrators through a medium are synthesized, thereby specifying the component, which is included in the reception signal, of the seismic wave transmitted from the desired seismic vibrator. By utilizing the property that the cross-correlation value between pseudo random code signals of different patterns is smaller than the autocorrelation value between the same pseudo random code signals, the receiver can separate the reception signal corresponding to the seismic wave transmitted from the specific seismic vibrator from the received reception signal. The pseudo random code signal is a representation of the sign of the pseudo random code by the phase of the sine wave, and the sign 0 and the sign 1 of the pseudo random code are respectively represented by the sine wave of 0 to 360 degrees and the sine wave of −180 to 180 degrees.
However, it was found that distinguishing the pseudo random code signals by pattern could not be sufficiently performed when this technique was applied to the seismic survey. There are some conceivable reasons such as a fact that the cross-correlation value includes noise of about 2% to 3% of the peak value of the autocorrelation value. For this reason, when using such a technique, it is difficult to output the seismic waves simultaneously even when the plurality of seismic vibrators are prepared, and a long measurement time is required to receive the signals from all of the seismic vibrators, and so there is a problem that it is difficult to obtain a high S/N ratio.